There is a move towards high data rate transmission in wireless communication systems. Efforts are now directed towards developing wireless systems that provide data rates on the order of 10 Mb/s. To achieve these high rates there has been a search for modulation schemes more efficient than M-ary PSK.
Different forms of orthogonal modulation have been considered and adopted by various standards committees. A form of orthogonal modulation was adopted in the IS-95 standard as the modulation technique used for the return link. J=log2 M bits are transmitted per symbol and one of M orthogonal binary sequences known as the Hadamard-Walsh functions is selected for transmission. This technique is called M-ary orthogonal keying (MOK).
One of the main advantages of MOK is the ability to demodulate the signals noncoherently with minimal performance degradation. Compared to differential PSK, noncoherent detection of MOK has a much better performance and so it has been considered by a number of different standards. MOK may have better BER performance than BPSK, which is due to the embedded coding properties of MOK. For the above mentioned advantages, MOK is considered a great choice for reverse link communications, where there is usually no pilot or reference signal to assist coherent detection.
For Additive White Gaussian Noise (AWGN) channel, the performance difference between noncoherent and coherent detection of MOK is less than 1 dB for M>2. For fast fading channels, the performance of noncoherent detection may be several dBs worse than that of coherent detection. Practical systems, especially terrestrial mobile communication systems with low SNR and severe fading, concatenate MOK with a form of forward error correction, usually convolutional codes. The present invention could work for MOK with any kind of outer coding. That is, the outer code need not be a convolutional code, but any code that can be decoded while generating soft decoder outputs. Optimum reception of this scheme is to implement one maximum likelihood receiver, which is very complex to implement. For near optimum reception of this coded system, soft detection information should be exchanged between the different blocks of the receiver.